A lapse rate is the rate of temperature change with height. The faster
the temperature decreases with height, the "steeper" the lapse rate and
the more "unstable" the atmosphere becomes. Lapse rates are typically
displayed in ranges from 850-500-mb (4,500-18,000-ft above sea level)
and 700-500-mb (10,000-18,000-ft above sea level).

Lapse rates are shown in terms of degrees Celcius change per
kilometer in height. Values less than 5.5-6.0 degrees C/km ("moist"
adiabatic) represent "stable" conditions, while values near 9.5 degrees
C/km ("dry" adiabatic) are considered "absolutely unstable." In between
these two values, lapse rates are considered "conditionally unstable."
Conditional instability means that if enough moisture is present, lifted
air parcels could have a negative LI (lifted index) or positive CAPE.